Liquid manure spreaders are used in the transportation of liquid manure and in the application of liquid manure to fields. Liquid manure is generally comprised of a slurry of liquid and solid animal waste produced from animal husbandry farming operations. A liquid manure spreader typically comprises a tank mounted upon a frame having one or more axles and having hitch means for attachment to the drawbar of a towing vehicle, typically a farm tractor. Due to the increasing size of farming operations, there has been a trend in the industry towards large capacity liquid manure spreaders that can handle, for example, in excess of 8000 gallons, more typically 10,000 gallons. These large capacity liquid manure spreaders are quite heavy and create significant soil compaction. In addition, recent road weight allowance limits enacted in legislation have mandated a maximum weight per axle within a certain prescribed distance. These factors have caused manufacturers of large capacity liquid manure spreaders to increase the number of axles, typically to three or four axles for very large capacity units.
In order to accommodate the large capacity tank and the increased number of axles, large capacity manure spreaders also have a long length. Since the axles are typically spaced apart along the length of the spreader, the long length makes the spreaders difficult to turn. It is desirable that the spreader is able to turn within a short turning radius. In response to this difficulty, manufacturers of large capacity spreaders have adopted steering systems that permit the rear axle or front and rear axles to turn in response to a change of direction of the towing vehicle. U.S. Pat. Nos. 5,364,116 and 5,595,397 provide examples of such systems. However, these systems are complicated, expensive, and difficult to maintain in the highly corrosive and dirty environment in which liquid manure spreaders are used.
The distribution of weight on to the drawbar of the towing vehicle is also an important factor in maneuvering the spreader. When turning, the contents of the tank move opposite to the direction in which the spreader is turned, making it difficult to steer the spreader. Also, due to the overall weight of a large-capacity spreader, the spreader tends to travel in a straight line during a turn, pushing the rear of the towing vehicle during a turn; this problem is exacerbated with insufficient weight transfer to the drawbar of the towing vehicle. The contents of the tank tend to slosh during transport, causing a jarring movement of the spreader and the towing vehicle due to oscillatory weight distribution. When travelling uphill, the contents of the tank tend to move towards the rear of the spreader, decreasing the weight distribution on to the drawbar and reducing the traction of the towing vehicle. This makes it difficult to pull the spreader uphill, a problem exacerbated by the overall weight of a large capacity spreader. EP 0 202 037, filed by Brian Lund on Apr. 21, 1986 and granted Jul. 2, 1990, attempts to address this problem in a smaller spreader by partitioning the tank and draining first the rear partition, then the front partition. A system is provided for transferring the contents of the front partition to the rear by means of air pressure. However, this system is complicated, expensive and not particularly well suited to large capacity spreaders.
In order to mitigate the tendency of the spreader to tip during turning, it is desirable to maintain a low center of gravity. In the prior art, the complicated steering systems raise the overall height of the spreader, necessitating the use of an oblate or T-shaped tank to provide clearance for the steering system while lowering the center of gravity of the spreader. However, when travelling along the side of a hill perpendicular to the slope of the hill, the center of gravity of a T-shaped tank moves significantly upwardly and to one side, exacerbating the pre-existing susceptibility to tipping of these spreaders. In addition, these types of tanks are expensive to construct and it is difficult for the farmer to predict the volume of material remaining in the tank as the contents are being spread, due to the non-uniform relationship between the liquid level in the tank and the volume of the tank.
The need therefore still exists for an improved large capacity liquid manure spreader.